Very small. They served on Naval ships and occasionally served in shore parties along the coasts.

The single most notable occasion could be that a battalion of marines scrounged up from the Navy Yard was with the Union army at First Manassas along with a handful of Regulars battalions were the only fully trained formations in either army. But even then they had many new recruits and did not give an outstanding account of themselves.

There was a CSAMC and they did much the same and served on the few CSN ships, including manning guns, and occasionally manned guns in coastal forts. Unfortunately many of their records, along with those of the CSN were lost in the fires in Richmond and Petersburg in April 1865.

Donnybrook: The Battle of Bull Run